When aluminum is smelted, a dross is skimmed off the surface of the motel metal. This dross contains aluminum oxide, aluminum nitride, and various other metal impurities, including a substantial amount of entrained metal. The metal is often 25 to 80% by weight of the dross, averaging about 50%. Some other metals including silicon, iron, lead and cadmium are found in the dross in smaller amounts which vary greatly with the source of the dross. The dross usually include some salt, in the form of principally sodium and potassium chloride. The amount of such salt varies greatly, and can be in the range 10 to 50 weight percent of the dross. It is desirably to recover as much of the aluminum metal as possible from the dross both to obtain usable product and to reduce the volume of waste product. While the dross of secondary smelters is not valuable enough to ship back to the primary smelters, practically all dross is processed to recover aluminum metal and the slag residue after processing is transported to landfills for disposal.
A commercial process of aluminum recovery involves crushing, sizing, and melting the metal from the dross under a salt flux, which usually consists of a eutectic mixture of sodium chloride and potassium chloride plus one to five percent cryolite or calcium fluoride. Flux is a substance that causes other substances to melt more readily by dissolving their oxides or surface impurities. This commercial aluminum recovery usually takes place in a rotating furnace with a mixture of the dross and salt flux in the ratio of one part dross to as much as one part salt flux. The mixture is heated to above the melting point of the aluminum and salt flux, while the aluminum oxide remains solid. Molten salt flux selectively wets the oxide of the dross and promotes separation of the molten metallic aluminum from the solid oxide fraction. At the same time, the aluminum is protected from oxidation by the salt flux. The denser molten metal sinks. After melting is completed, the floating mixture of oxide and salt can be poured from the furnace for disposal in a land fill, and the separated metal layer can be recovered free of salt and oxide.
Primary and secondary smelting plants in the United States generate over 200,000 tons per year of dross, with estimates ranging from 300,000 to 500,000 tons per year. Salts added during processing can be leached from the residues by rainfall and the leachate can contaminate both surface and ground waters. In addition, the salts volatilize during processing, forming corrosive fumes that are discharged into the atmosphere. These environmental problems could result in restrictions on the dumping of the residues or the use of the salt flux. Either action could have the effect of reducing the recovery of aluminum.
There are two principal types of plasma torches. If only one electrode is in the torch, and the other electrode is the material to be heated, the plasma torch is of the transferred arc type. If both electrodes are within the plasma torch, the torch is said to be non-transferred.
EP 0 322 207 A2 filed Dec. 21, 1988 describes the batch recovery of non-ferrous metals, particularly aluminum, from dross without utilizing the conventional gas fired treatment with a molten salt bath, and is carried out in a rotary furnace utilizing using plasma torch heating. An arc plasma torch, preferably non-transferred, is used to heat air, nitrogen, argon, or other gases or mixtures, which heated gas is then directed upon the rotating dross charge to melt and separate the aluminum metal from the oxide portion of the dross. When separation is accomplished, the recovered metal is tapped off. Following this, the dross residue is scraped from the reactor. The amount of residue is less than in the conventional treatment where salt is added, thereby decreasing the severity of disposal problems.
U.S. Pat. No. 4,877,448 describes a process for recovering aluminum metal and aluminum oxide from dross in a furnace using plasma torch heating. Air is heated in the plasma torch to melt the charge, preferably in a rotating drum furnace. Some of the metal is oxidized, producing a portion of the heat required. After the torch is turned off, molten aluminum is discharged by tilting the drum. Residual aluminum oxide is scraped from the drum.
U.S. Pat. No. 4,997,476 describes a process for recovering aluminum and aluminum oxide in a rotary furnace using plasma torch heating. A variety of arc gases, preferably air, is heated in the plasma torch and directed upon the rotating dross charge to separate and agglomerate the free aluminum from the dross residue without need for the conventional molten salt bath. When separation is accomplished, the recovered metal is tapped off. After each run, or after two or three runs, the aluminum oxide dross residue is scraped from the walls of the furnace. The amount of residue is less than in the conventional treatment wherein salt is added, thereby decreasing the severity of disposal problems.
J. Meunier, et al., "Aluminum Recovery from Dross: Comparison of Plasma and Oil-fired Rotary Furnaces", 9th International Symposium on Plasma Chemistry (ISPC-9), Italy (1989) compares the batch aluminum recovery from dross in non-transferred arc plasma torch and oil-fired furnaces. Both nitrogen and air were tested as the plasma gas for the plasma furnace, and air was used for the oil-fired furnace; fluxing salts were used only in the oil-fired furnace. The aluminum was poured into pigs and weighed and the residue in the furnaces was collected and weighed for mass balances. The plasma torch gave better recoveries of aluminum when air was used in both furnaces. The recovery of aluminum was improved still further, to 94.5%, using nitrogen in the plasma torch. Equivalent energy efficiencies of close to 30% were measured for both processes.
D. Montagna, "A Fluxless Method for Reclaiming Aluminum from Dross, " U.S. Bur. of Mines Rep. Invest., RI 8288 (1978) describes a batch salt-free fluxless method for reclaiming metallic aluminum from aluminum dross. The dross is smelted under argon, nitrogen, or carbon dioxide in an externally heated electric-pot furnace. This pot furnace does not contain a plasma arc, and requires a stirring mechanism to separate the melted metal from solid oxide. The pot furnace is tilted to pour out the uppermost molten aluminum layer. The bottom fluxless residue layer can be resold to steel companies for hot-topping of steel ingots or discarded. Use of argon led to recoveries equal to that in commercial rotary-type furnaces with salt flux, but the amount of residue generated is less.